Field of the Invention
The present invention relates to a heterocyclic compound and an organic light emitting display device comprising the same, and more particularly, to a heterocyclic compound which is capable of reducing the operating voltage of an organic light emitting display device and improving its efficiency and lifetime and an organic light emitting display device comprising the same.
Discussion of Related Art
With the development of multimedia, panel displays are becoming more and more important. Accordingly, a variety of panel displays such as liquid crystal display (LCDs), plasma display panels (PDPs), field emission displays (FEDs), organic light emitting display devices, and the like are put to practical use.
Among them, the organic light emitting display devices are advantageous in that they can be formed on a flexible transparent substrate, such as plastic, can be driven at a low voltage of 10 V or less, have relatively low power consumption, and excellent color sensitivity, as compared to a plasma display panel or inorganic light emitting diode display. Further, the organic light emitting display device can represent three colors of green, blue and red, and thus is drawing a great deal of attention as a next-generation full-color display device.
An organic light emitting display device can be formed by sequentially forming an anode, a hole injection layer, a hole transport layer, a light emitting layer, and electron transport layer, an electron injection layer, and a cathode. For a luminescent material, excitons are formed by the recombination of electrons and holes injected from the two electrodes. Singlet excitons and triplet excitons are involved in fluorescence and phosphorescence, respectively. In recent years, there is a growing trend that phosphorescent materials are replacing fluorescent materials. For a fluorescent material, singlet excitons, which make up only 25% of all excitons formed in the light emitting layer, are used to produce light, and triplet excitons, which make up 75% of the excitons, are mostly lost and transformed into heat. Phosphorescent materials, in contrast, have a light emission mechanism for converting both singlet and triplet excitons into light.
A light emitting process of a phosphorescent material will be discussed briefly. Holes injected from the anode and electrons injected from the cathode meet in a host material of the emission layer. Though a hole and an electron may be paired in a dopant in some cases, a large amount of holes and electrons meet in the host in most cases due to high concentration of the host. At this point, the singlet excitons formed in the host transfer energy to the singlets or triplets of the dopant, while the triplet excitons transfer energy to the triplets of the dopant.
Since the excitons transferred to the singlets of the dopant are transferred to the triplets of the dopant by intersystem crossing, the first destination of all the excitons is a triplet level of the dopant. The thus-formed excitons are transferred to the ground state, and emit light. If the triplet energy of the hole transport layer or electron transport layer adjacent to the front and back of the light emitting layer is less than the triplet energy of the dopant, backward energy transfer takes place from the dopant or host to these layers, and this leads to an abrupt decrease in efficiency. Accordingly, the triplet energy of the hole/electron transport layers, as well as the host material of the light emitting layer, plays a very important role in phosphorescent devices.
For efficient energy transfer from the host to the dopant, the triplet energy of the host must be greater than the triplet energy of the dopant. For green light emission, the triplet energy of the host must be equal to or greater than 2.5 eV as long as the triplet energy of the dopant is 2.4 eV or greater, in order to facilitate energy transfer. However, materials with high triplet energy cause deteriorations of the device, including a decrease in device efficiency and a voltage rise. Materials with low thermal stability and low electric stability can decrease the lifetime of the device. Accordingly, there is a need for the development of novel phosphorescent materials with superior thermal stability and superior electric stability.